Theoretical Study of the Pseudo-Jahn−Teller Effect in the Edge-Sharing Bioctahedral Complex Mo₂(DXylF)₂(O₂CCH₃)₂(μ₂-O)₂

A study of the D2h to C2h pseudo-Jahn−Teller distortion in the edge-sharing bioctahedral complex Mo2(DXylF)2(O2CCH3)2(μ2-O)2 is presented. We have performed extensive density functional theory (DFT) and complete active space self-consistent field (CASSCF) calculations. For both the full target complex and a model derived by replacing xylyl and methyl groups with hydrogens we observe that the central Mo2(μ2-O)2 motif displays C2h rather than D2h symmetry. Analytical CASSCF frequency calculations prove that the rhomboidal distortion of the complex from D2h to C2h is due to a vibronic mixing of the ground electronic state and a low-lying πδ* excited state

Photoisomerization in a Platinum−Amido Pincer Complex: An Excited-State Reaction Pathway Controlled by Localized Ligand Photochemistry

Computational investigations of the electronic spectroscopy and photochemical isomerization in the complex (bis(8-quinolinyl)amido)PtMe₂I are presented. Time-dependent density functional theory, in conjunction with the polarizable continuum solvent model, reproduce the experimental spectra for the mer and fac isomers well. The nature of the initially populated states for the mer isomer are ππ* in nature and localized on the BQA ligand. Geometry optimization shows that the system relaxes in the excited manifold to a fac-like geometry in the S₁ electronic state. Complete active space self-consistent field (CASSCF) calculations show that there exists a sloped conical intersection that connects the excited- and ground-state fac species, allowing for radiationless deactivation in fac-like geometries

Photostereochemistry and Photoaquation Reactions of [Cr(tn)₃]³⁺: Theoretical Studies Show the Importance of Reduced Coordination Conical Intersection Geometries

We have performed TD-DFT and CASSCF calculations to understand the spectroscopy and reactive photochemistry of the [Cr­(tn)₃]³⁺ complex. Our results show that, after population of a quartet ligand field excited state, the system relaxes by dissociation of a Cr–N bond to reach a quasi-trigonal bipyramid five-coordinate species that is a conical intersection connecting the excited and ground quartet manifolds. Nonadiabatic relaxation through these leads to square pyramidal structures that can coordinate water and account for the observed monoaquated photoproducts. Such features are also present on the potential energy surfaces of these photoproducts and account for the range of experimentally observed photostereoisomers of the photoaquation reactions

A Conformationally Flexible, Urea-Based Tripodal Anion Receptor: Solid-State, Solution, and Theoretical Studies

Tripodal tris(urea) cationic receptors 1 and 2 containing p-tolyl or octyl substituents, respectively, have been synthesized, and their association behavior with anionic guests has been studied via a variety of methods. The receptors are based around a hexasubstituted aryl core and contain both urea and pyridinium functionalities. For 1:1 complexes, anions reside within the central cavity of the host species, held by hydrogen bonds from both NH and CH donors. The following host−anion complexes have been characterized by X-ray crystallography:  1−(Br)3, 1−(PF6)3·2(CH3)2CO, and 1−(NO3)1.5(PF6)1.5. Each structure contains the receptor in a significantly different geometry, highlighting the anion-dependent conformational flexibility of 1. Solution 1H NMR spectroscopic titrations have shown the two host species to display significant affinity for both halides and hydrogen sulfate and strongly suggest the persistence of CH···X- interactions despite the presence of “stronger” NH donor groups. Variable-temperature 1H NMR studies on the more soluble octyl derivative 2 show that there is a distinct change in conformation associated with the formation of a 1:1 host/guest complex. Computations using density functional theory (with the B3LYP functional) have been employed to aid in understanding the geometry of the 1:1 host/chloride complexes of 1 and 2. These experiments suggest that the lowest energy conformation for 1−Cl is one in which the ureidopyridinium arms are orientated upward forming a cavity that is sealed by CH···π interactions, effectively forming a unimolecular capsule, whereas for 2 a less symmetrical “2-up, 1-down” geometry is favored

A Conformationally Flexible, Urea-Based Tripodal Anion Receptor: Solid-State, Solution, and Theoretical Studies

Tripodal tris(urea) cationic receptors 1 and 2 containing p-tolyl or octyl substituents, respectively, have been synthesized, and their association behavior with anionic guests has been studied via a variety of methods. The receptors are based around a hexasubstituted aryl core and contain both urea and pyridinium functionalities. For 1:1 complexes, anions reside within the central cavity of the host species, held by hydrogen bonds from both NH and CH donors. The following host−anion complexes have been characterized by X-ray crystallography:  1−(Br)3, 1−(PF6)3·2(CH3)2CO, and 1−(NO3)1.5(PF6)1.5. Each structure contains the receptor in a significantly different geometry, highlighting the anion-dependent conformational flexibility of 1. Solution 1H NMR spectroscopic titrations have shown the two host species to display significant affinity for both halides and hydrogen sulfate and strongly suggest the persistence of CH···X- interactions despite the presence of “stronger” NH donor groups. Variable-temperature 1H NMR studies on the more soluble octyl derivative 2 show that there is a distinct change in conformation associated with the formation of a 1:1 host/guest complex. Computations using density functional theory (with the B3LYP functional) have been employed to aid in understanding the geometry of the 1:1 host/chloride complexes of 1 and 2. These experiments suggest that the lowest energy conformation for 1−Cl is one in which the ureidopyridinium arms are orientated upward forming a cavity that is sealed by CH···π interactions, effectively forming a unimolecular capsule, whereas for 2 a less symmetrical “2-up, 1-down” geometry is favored

A Conformationally Flexible, Urea-Based Tripodal Anion Receptor: Solid-State, Solution, and Theoretical Studies

Tripodal tris(urea) cationic receptors 1 and 2 containing p-tolyl or octyl substituents, respectively, have been synthesized, and their association behavior with anionic guests has been studied via a variety of methods. The receptors are based around a hexasubstituted aryl core and contain both urea and pyridinium functionalities. For 1:1 complexes, anions reside within the central cavity of the host species, held by hydrogen bonds from both NH and CH donors. The following host−anion complexes have been characterized by X-ray crystallography:  1−(Br)3, 1−(PF6)3·2(CH3)2CO, and 1−(NO3)1.5(PF6)1.5. Each structure contains the receptor in a significantly different geometry, highlighting the anion-dependent conformational flexibility of 1. Solution 1H NMR spectroscopic titrations have shown the two host species to display significant affinity for both halides and hydrogen sulfate and strongly suggest the persistence of CH···X- interactions despite the presence of “stronger” NH donor groups. Variable-temperature 1H NMR studies on the more soluble octyl derivative 2 show that there is a distinct change in conformation associated with the formation of a 1:1 host/guest complex. Computations using density functional theory (with the B3LYP functional) have been employed to aid in understanding the geometry of the 1:1 host/chloride complexes of 1 and 2. These experiments suggest that the lowest energy conformation for 1−Cl is one in which the ureidopyridinium arms are orientated upward forming a cavity that is sealed by CH···π interactions, effectively forming a unimolecular capsule, whereas for 2 a less symmetrical “2-up, 1-down” geometry is favored

A Conformationally Flexible, Urea-Based Tripodal Anion Receptor: Solid-State, Solution, and Theoretical Studies

Tripodal tris(urea) cationic receptors 1 and 2 containing p-tolyl or octyl substituents, respectively, have been synthesized, and their association behavior with anionic guests has been studied via a variety of methods. The receptors are based around a hexasubstituted aryl core and contain both urea and pyridinium functionalities. For 1:1 complexes, anions reside within the central cavity of the host species, held by hydrogen bonds from both NH and CH donors. The following host−anion complexes have been characterized by X-ray crystallography:  1−(Br)3, 1−(PF6)3·2(CH3)2CO, and 1−(NO3)1.5(PF6)1.5. Each structure contains the receptor in a significantly different geometry, highlighting the anion-dependent conformational flexibility of 1. Solution 1H NMR spectroscopic titrations have shown the two host species to display significant affinity for both halides and hydrogen sulfate and strongly suggest the persistence of CH···X- interactions despite the presence of “stronger” NH donor groups. Variable-temperature 1H NMR studies on the more soluble octyl derivative 2 show that there is a distinct change in conformation associated with the formation of a 1:1 host/guest complex. Computations using density functional theory (with the B3LYP functional) have been employed to aid in understanding the geometry of the 1:1 host/chloride complexes of 1 and 2. These experiments suggest that the lowest energy conformation for 1−Cl is one in which the ureidopyridinium arms are orientated upward forming a cavity that is sealed by CH···π interactions, effectively forming a unimolecular capsule, whereas for 2 a less symmetrical “2-up, 1-down” geometry is favored

Biomolecular Mode of Action of Metformin in Relation to Its Copper Binding Properties

Metformin (Metf), the most commonly used type 2 diabetes drug, is known to affect the cellular housekeeping of copper. Recently, we discovered that the structurally closely related propanediimidamide (PDI) shows a cellular behavior different from that of Metf. Here we investigate the binding of these compounds to copper, to compare their binding strength. Furthermore, we take a closer look at the electronic properties of these compounds and their copper complexes such as molecular orbital interactions and electrostatic potential surfaces. Our results clearly show that the copper binding energies cannot alone be the cause of the biochemical differentiation between Metf and PDI. We conclude that other factors such as p<i>K</i>_a values and hydrophilicity of the compounds play a crucial role in their cellular activity. Metf in contrast to PDI can occur as an anion in aqueous medium at moderate pH, forming much stronger complexes particularly with Cu^II ions, suggesting that biguanides but not PDI may induce easy oxidation of Cu^I ions extracted from proteins. The higher hydrophobicity and the lack of planarity of PDI may further differentiate it from biguanides in terms of their molecular recognition characteristics. These different properties could hold the key to metformin’s mitochondrial activity because they suggest that the drug could act at least in part as a pro-oxidant of accessible protein-bound Cu^I ions

Modular Approach to Selected Configuration Interaction in an Arbitrary Spin Basis: Implementation and Comparison of Approaches

A modular selected configuration interaction (SCI) code has been developed that is based on the existing Monte-Carlo configuration interaction code (MCCI). The modularity allows various selection protocols to be implemented with ease and allows for fair comparison between wave functions built via different criteria. We have initially implemented adaptations of existing SCI theories, which are based on either energy- or coefficient-driven selection schemes. These codes have been implemented not only in the basis of Slater determinants (SDs) but also in the basis of configuration state functions (CSFs) and extended to state-averaged regimes. This allows one to take advantage of the reduced dimensionality of the wave function in the CSF basis and also the guarantee of pure spin states. All SCI methods were found to be able to predict potential energy surfaces to high accuracy, producing compact wave functions, when compared to full configuration interaction (FCI) for a variety of bond-breaking potential energy surfaces. The compactness of the error-controlled adaptive configuration interaction approach, particularly in the CSF basis, was apparent with nonparallelity errors within chemical accuracy while containing as little as 0.02% of the FCI CSF space. The size-to-accuracy was also extended to FCI spaces approaching one billion configurations

Excited-State Absorption of Conjugated Polymers in the Near-Infrared and Visible: A Computational Study of Oligofluorenes

Excited-state properties of conjugated polymers play a central role in applications ranging from organics-based photovoltaics to nonlinear photonics. From a theoretical and computational point of view, however, an accurate first-principles description poses a formidable task. Typical molecule sizes go well beyond the size limits for which highly reliable wave function based electronic-structure methods can be applied. In the present work, we demonstrate that nonlinear-response density functional theory can be used to accurately model the excited state absorption process in an important class of conjugated materials. We compute transitions between up to 100 excited states for fluorene oligomers containing up to about 100 conjugated atoms. Furthermore, we demonstrate that this approach can explain the nature of absorption bands in the ESA in near-infrared and visible spectral range. These systems are large enough that we approach the polymer limit in terms of electronic properties of excited states. The results obtained are in good agreement with available experimental data